In an electrophotographic copying machine, each of the sheets of copy paper on which images of an original are made is transported along a predetermined path through various stations such as feeding rollers, transfer charges, a photoreceptor drum, heat rollers and so on.
During the time the copy paper is being transported through the various stations, the copy paper may be caught at some places in the stations so that the copy paper may become jammed in the copying machine.
One conventional jam detecting device for detecting such jams employs one or more paper detecting devices disposed at the various stations as described above so as to detect the travelling copy paper and timer means for defining the periods of operation of each of the detecting devices corresponding to the positions at which each of the detecting devices is located. In such device, each of the paper detecting devices acts to detect whether or not the leading edge of a sheet of copy paper passes through a predetermined position in a given period in response to instructions fed from the timer means, thereby detecting normal transportation of a sheet of copy paper by a signal from the paper detecting device showing the passage of the sheet of copy paper.
However, in a conventional paper jam detecting device as described above, where a sheet of copy paper is jammed immediately after one paper detecting device has detected the passage of the leading edge of the copy paper, only the other paper detecting device disposed downstream of said one paper detecting device can detect said paper jam.
Therefore, the conventional paper jam detecting device has the disadvantages that the position where a copy paper is jammed can not be detected accurately and the detection of the paper jam is undesirably delayed since it requires a long time before the other paper detecting device disposed downstream detects said paper jam.
In order to avoid such disadvantages as described above, there could be provided an arrangement in which a larger number of the paper detecting devices are located with a shorter distance between two paper detecting devices adjacently disposed. In such an arrangement, it would be necessary to provide a lot of electromagnetic timers to define the operation periods of the respective paper detecting devices. However, the provision of a lot of timers requires a lot of space and it becomes troublesome not only to set the respective timers for different periods of time, but also to maintain them.
In addition, in order to avoid occurrence of errors in detecting the paper jam, each of the timers must operate in synchronism with the motions of the drive mechanism of the paper feeding device, thereby making it difficult to make the paper jam detecting device.
Another paper jam detecting device is disclosed in U.S. Pat. No. 4,084,900 wherein a timer is adapted to start counting time with respect to the trailing edge of a sheet of copy paper travelling in a predetermined path, while a paper detecting device is provided in the vicinity of the paper discharge chute of the copying machine so as to detect the sheet of passage of the trailing edge of the copy paper, whereby a jam detecting signal can be obtained from the paper detecting device if no paper is detected within the period of time counted by said timer. In this arrangement, the disadvantage inherent in the device for detecting the leading edge of a sheet of copy paper can be eliminated.
In this jam detecting device, however, it is necessary to provide a timer for counting a long period of time, which is expensive, and it takes long time before the paper jam is detected since the timer is adapted to start counting of time in response to the operation of the cutting device for cutting a roll paper and to continue counting of time until the leading edge of a sheet of copying paper passes the discharge chute.
A paper jam detecting device is also disclosed in U.S. Pat,. No. 3,833,896 wherein a sheet of copy paper is detected by means of a detecting switch MS disposed near the paper discharge chute of a copying machine. In this paper jam detecting device, a first period is set in the first timer in synchronization with the leading edge of the sheet of copy paper and a paper jam is detected if the leading edge of the copy paper does not reach the paper detecting device within said first period.
In turn the second timer is adapted to start when the leading edge of the sheet of copy paper reaches the switch SM within the first period, whereby a paper jam is detected if no copy paper passes the switch MS before the second timer completes the counting of the period time assigned thereto.
However, this paper jam detecting device also has a problem in practical use that it takes a long time, 5 minutes for example, to detect a paper jam after feed of the copy paper begins, since the copy paper is detected only by the detecting member located near the paper discharge chute.
In addition, the second timer must be set for a period of time corresponding to the largest size of the copy paper used in the copying machine in which various sizes of sheets of copy paper are available, so that much time is lost when a short sheet of copy paper is used.